1. Field of the Invention
The present invention relates to spectral filters and methods of making spectral filters for filtering infrared, visible and ultraviolet radiation. More particularly, the present invention relates to high-pass, low-pass and band-pass filters constructed by coating the channels of a nanochannel glass array with a reflective material.
2. Description of the Related Art
Many techniques exist currently for filtering infrared, visible and ultraviolet radiation. Among these are: monochromators (grating or prism types), resonators (for example, Fabry-Perot type), multilayer dielectric thin films on appropriate substrates, films or bulk materials having appropriate dielectric functions, absorbing colored filters, scatter filters, at al., In practice, all of these techniques are adversely affected, to varying degrees, by environmental factors such as heat, humidity, vibrations, etc.; no rugged filtering technique, which would be relatively insensitive to changes in these environmental factors, exists at present.
At 100 .mu.m and longer wavelengths, filters based on arrays of metallic waveguides have been fabricated. Such high-pass filters for the far-infrared spectral region are described in the following publications: Fritz Keilmann, Int. J. of Infrared and Millimeter Waves 2, 259 (1981); T. Timusk and P. L. Richards, Appl. Optics 20, 1355 (1981) and P. G. Huggard, M. Meyringer, A. Schilz, K. Goller, and W. Prettl, Appl. Optics 33, 39 (1994). These filters are very rugged, generally consisting of a single piece of perforated metal or grid; they are relatively light and compact, are not very sensitive to environmental factors such as heat, humidity, and mechanical vibrations, and are not affected by reasonable misalignments. Their spectral properties are relatively easy to control by modifying the geometrical parameters of the waveguides. Besides the ruggedness, far-infrared filters based on arrays of waveguides have several additional advantages over other types of filters. For a closely packed array of waveguides, the transmission of these filters greatly exceeds the fraction of open area (the total cross-sectional area of the waveguides divided by the total area covered by the array). Due to a resonance involving neighboring waveguides, the transmission can approach unity under favorable conditions, thus exceeding that obtained by other filtering methods. Also, unlike the cases of other filtering methods, the cutoff frequency is insensitive to the propagation direction of the incident radiation, while the transmission efficiency decreases only very gradually as the propagation direction deviates from the normal to the plane of the waveguide array.
Filters based on metallic waveguides are much easier to manufacture for longer wavelengths because the cutoff frequency is determined by the waveguide diameter. For a hollow cylindrical waveguide having circular cross-section and walls with infinite conductivity (an approximation valid for most metals in the far-infrared), the cutoff frequency is simply .nu..sub.c .varies. 0.586/d, where d is the diameter of the waveguide. The aspect ratio of the waveguide, t/d, where t is the length of the waveguide, determines the rolloff properties. Formed metallic grids have been used most commonly in the far-infrared, either singly or stacked; individually, such grids have an aspect ratio (t/d)&lt;1, giving them relatively poor filtering properties. For larger aspect ratios at long enough wavelengths (longer than 300 .mu.m), it is possible to manufacture a filter consisting of an array of waveguides with an aspect ratio greater than 1 by drilling holes in a piece of metal. However, such techniques cannot be extended into the near-infrared, visible, and ultraviolet regions because, in order to have cutoff frequencies in these spectral regions, holes with d between 10 and 0.1 .mu.m and an aspect ratio (t/d)&gt;1 are required. Arrays of such small holes are well beyond the reach of the manufacturing techniques used for the far-infrared filters. Since no economical alternative for manufacturing the required arrays of 10 to 0.1 .mu.m holes existed previously, near-infrared, visible, and ultraviolet filters based on this concept could not be made until now.